I am a developer of Hspice and currently in charge of developing
and maintaining the W-element and the internal field solver.
I would like to clarify the four Hspice issues which have been
actively discussed:
1) Accuracy of the field solver for computing the skin-effect
matrix Rs.
2) Discussion of the inclusion of the imaginary term for Rs:
SQRT(f)*Rs [eq1] vs. SQRT(f)*Rs*(1+j) [eq2].
3) Discrepancy between the transient and ac analysis results for
the cases with non-zero Gd values.
4) Discrepancy between the transient analysis results using
a single long line versus several short lines.
(1) It is clearly stated in the Hspice manual that the computation
of Rs does NOT account for any proximity or edge effects; hence,
the resulting value is significantly smaller than the actual value.
This is consistent with the computed data by Richard Mellitz.
We are currently working on implementing a new solver which
accurately models the skin, proximity and edge effects.
(2) The main issue for this second problem is that while including
the imaginary term [eq2] models the skin effect (marginally) better
at high frequencies, it may introduce a significant error at low
frequencies. Some people, especially those working on high
frequency applications, are accustomed to use [eq2] while others
prefer [eq1].
I prefer to use [eq2] since it is mathematically more valid solution
at the high frequency range where the SQRT(f) dependency is valid,
and furthermore, both equations are not accurate at low frequencies
anyway. I ran some test cases using both equations and observed no
significant differences in ac analysis. However, the accuracy of
the transient analysis is slightly degraded when [eq1] is used.
Nonetheless, we will change the implementation so that you can
optionally exclude the imaginary term.
(The ultimate solution to this Rs problem, as it has been pointed
out by Michael Tsuk, could be to use a better modeling equation
or even tabular data.)
(3) The difference in attenuation between transient and ac analyses
with non-zero Gd values is a rather subtle problem. The cause of
this problem is mainly due to the limitation of the dielectric
model equation based on f*Gd as has been pointed out by Dmitri.
(4) The discrepancy between the transient results using a single
line versus several cascaded short lines is mainly due to the
limitations on the difference approximation routine used in
W-element for the frequency-dependent cases. Recently, we have
significantly improved the difference approximation routine, and
and this discrepancy has been eliminated. (No risetime parameter
setting is necessary anymore.)
All of these improvements will be available for the next release
of Hspice (version 99.4). We will soon make a beta version available
(approximately 1 month); however, the new solver mentioned in (1)
won't be implemented in this beta version.
I hope this note clarifies all the issues related to the W-element
and field solver in Hspice.
I apologize for a somewhat late response.
Regards,
___________________________________________________________________
Kyung Suk (Dan) Oh, Ph.D. (B) (510) 413-8197
Avant! Corporation (F) (510) 413-8080
46871 Bayside Parkway (e) dan_oh@avanticorp.com
Fremont, CA 94538
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